The present invention relates to a process for using biodegradable muds during the drilling of a well. The invention relates more particularly to a process for using several types of mud throughout the length of the well.
It is common practice to use a single drilling mud throughout the depth of the well, the specific properties of this mud being linked to the drilling temperature at the bottom of the well and the necessary ability of the mud to cool down, to lubricate, to plug the wall of the terrains passed through and to bring the cuttings up to the surface.
In order to reach terrains in which the temperatures at the bottom of the well reach 200xc2x0 C. and above, drilling muds must keep their physical properties intact, i.e. their capacity for lubrication, their resistance to oxidation, their carrying power for bringing the cuttings up to the surface, their density and their activity as filtrate-reducing agents. These muds must also contain sufficient basic reserves to neutralize the acidic species encountered during the drilling and generally found in deep layers, such as carbon dioxide or hydrogen sulphide. When terrains are reached containing these acidic species and reaching temperatures above about 100xc2x0 C., 70% of the volume of the mud required for the drilling has been used, since the deeper the drilling proceeds into the ground, the more the diameter of the well reduces. This results in about 70% by volume of poorly-employed mud, since in this first part of the well corresponding to the so-called surface drilling, the properties required may be less specialized than those required at the bottom of the well. In particular, since the drilling temperatures are low (below 100xc2x0 C.), the drilling fluid undergoes little transformation such as oxidation, hydrolysis or even decomposition.
It is thus a case of adapting the mud to the terrain passed through, which has a direct impact on the technical, physical and chemical properties of the muds selected, and on the order in which they are used, the consequence being economical, since the purchasing cost of the drilling muds may be reduced.
To add to this first problem are the problems of treating the cuttings at the surface, recovery of the mud by screening in order to recycle it, and storage of the cuttings. These problems, which are already difficult on land, become much more difficult in the case of offshore platforms, since the constraints with regard to environmental regulation are much stricter. Economically, it is more reasonable to dump these cuttings at sea rather than to transport them onshore for treatment, all the more since the volumes of cuttings are much greater, from 60 to 80% of the total volume of the cuttings dumped from the well, in the case of surface drilling. Dumping the cuttings at sea can thus depend on more than just the toxicity of the mud used or its biodegradability in the marine environment.
Among the muds currently used are water-based muds, oil-based muds and so-called replacement muds.
Oil-based muds comprise an oil base formed of crude petroleum or a hydrocarbon-based fraction, to which are added a certain number of additives for improving their rheological properties, their filtrate-reducing power and their basicity. Mention may be made of oils of xe2x80x9cdieselxe2x80x9d type or mineral oils with a low content of aromatic compounds.
Thus, U.S. Pat. No. 4,481,121 proposes oil-based muds containing from 75 to 95% by volume of gas oil or of a plant oil, in particular extract of pine resin or xe2x80x9ctall oilxe2x80x9d, paraffins with a melting point of between 76 and 120xc2x0 C. and from 5 to 25% by volume of a basic reserve consisting of diethanolamide and alkaline hydroxides.
However, such muds are highly toxic to the environment and biodegrade very slowly in aerobic and anaerobic media. Such pollution can pose a major problem if it is necessary to drill through a water table. Certain national legislations have already banned the use of these muds or the offshore dumping of drilling cuttings impregnated therewith.
Water-based muds using aqueous compositions obtained from polymers, polyols or polyglycerol, although being less polluting and biodegradable, are little used on account of their poor stability above 100xc2x0 C., their insufficient lubricant properties in deflected wells and the presence of water in clay-based terrains.
Replacement muds are used especially as a replacement for oil-based muds, which are often highly aromatic. They are formulated from non-toxic and non-aromatic hydrocarbon-based synthetic products.
A first type of replacement mud is obtained from linear or branched olefins, or, from poly(xcex1-olefins) whose performance characteristics are similar to those of oil-based muds, i.e. maintenance of the lubricant and rheological properties at high temperature, maintenance of good resistance to oxidation and maintenance of their non-toxic nature. They biodegrade slowly in aerobic medium and are relatively non-biodegradable in anaerobic medium (J. Steber et al., Offshore, September 1994). This problem is all the more critical when drillings are carried out at sea, in which it is common practice to dump the mud-impregnated cuttings. Piles of cuttings accumulate under the water, close to the platform, and an accumulation of non-biodegradable chemical compounds is observed in these piles. Inside these piles, since the medium is anaerobic, there is no possible biodegradation of these compounds, while at the surface of the piles, the medium being aerobic, there is slow degradation of these compounds. It is for this reason that certain legislations make provision to greatly limit or even ban the offshore dumping of these cuttings since they are relatively non-biodegradable.
The muds currently preferred consist of readily-biodegradable hydrocarbon-based products, among which are the synthetic esters as described in U.S. Pat. Nos. 4,374,737 and 5,232,910. These highly complicated muds are expensive since they have been designed to satisfy all drilling conditions, including drilling under extreme conditions such as withstanding a temperature above 200xc2x0 C., oxidation and acidic media.
Alongside these muds are reverse emulsions, which are less polluting since they are considered as being readily biodegradable, such as those described in PCT patent application WO 95/GB95/00680. These reverse emulsions consist of refined plant oils of high viscosity mixed with appropriate diluents to lower their viscosity. These muds based on reverse emulsions can be used irrespective of the temperature throughout the depth of the well. They are less toxic and more biodegradable than the previous muds, but have a very high cost price since they are fully refined and mixed with a diluent which is often expensive and various additives, in particular antioxidants, which are essential for correct behaviour of the emulsion at high drilling temperatures.
The aim of the present invention is to reduce the risks of pollution while at the same time substantially reducing the cost prices of the drilling operation as regards the total cost of the mud used. The invention is also directed towards reducing, or even eliminating, the cost of treating impregnated cuttings, since they contain a less toxic mud and are rapidly biodegradable, it being possible for these cuttings to be dumped into the sea as they are or stored on land.
A subject of the present invention is a process for using biodegradable drilling muds, characterized in that a biodegradable surface sludge which is stable up to about 100xc2x0 C., which has filtrate-reducing power which is adjustable from 2 to 13 cm3 depending on the amount of filtrate-reducing agent used, and which has an electrical stability of greater than or equal to 1000 volts, is introduced into the well at the start of drilling, and in that, when the temperature of the terrain passed through reaches 100xc2x0 C., the said surface sludge is replaced with a so-called bottom sludge which is stable up to temperatures above 200xc2x0 C., the volume of the said surface sludge representing from 60 to 80% by volume relative to the total volume of sludge used.
The term xe2x80x9csurface sludgexe2x80x9d means the mud which can be used from the start of drilling, and the term xe2x80x9cbottom sludgexe2x80x9d means the mud(s) used in the part of the well in which the specificity of the terrain requires specific chemical and physical resistance properties, such as heat resistance or the neutralization of the acidic species encountered.
In this process according to the invention, in contrast with conventional drilling processes during which only one type of mud is used, several muds, at least two, are used which are adapted to all the series of terrain passed through, for optimization of the cost price of the drilling and of the treatment of the cuttings, and a limitation of the pollution. In this process according to the invention, the first mud, referred to as the surface sludge, is selected so as to be very readily biodegraded. The mud changeover temperature is set at 100xc2x0 C., in order to give an order of magnitude, but this can be set slightly higher or lower according to the plan of progress of the well casing. However, 100xc2x0 C. also corresponds to the temperature at which the first mud, or surface sludge, becomes unstable and gradually decomposes over time.
The surface sludge is very rapidly biodegradable and relatively non-polluting to the terrains passed through and to the marine environment. It can be selected from the commercially available water-based muds and biodegradable reverse emulsions, the latter being preferred.
The surface sludge according to the invention consists of a reverse emulsion in which the oil/water ratio by volume ranges from 95/5 to 70/30.
The oil phase consists of a so-called eco-friendly oil, which is of low viscosity and is highly biodegradable in aerobic and anaerobic media. The major advantage of these eco-friendly oils lies in the simplification of the treatment of drilling cuttings impregnated with this oil phase. If these cuttings, or even the eco-friendly oil or the drilling mud itself, are dumped at sea, no pollution results therefrom.
The oil phase contains a majority of unrefined plant or animal natural oils and a minority of a diluent consisting of esters of these natural oils. Since these oils of animal or plant origin are unrefined, they contain 90% by weight, and preferably more than 95% by weight, of triglycerides. They are preferably chosen from the group consisting of sunflower oil, copra oil, soybean oil, rapeseed oil, castor oil, palm oil, palm kernel oil, groundnut oil, linseed oil, olive oil, corn oil, babassu oil, coconut oil and tallow oil.
These oils according to the invention consist of triglycerides corresponding to glycerol triesters of general formula I below: 
in which R1, R2 and R3, which may be identical or different, are chosen from saturated or unsaturated, linear aliphatic groups Ri consisting of 5 to 23 carbon atoms. These groups derived from aliphatic fatty acids can contain from 1 to 3 olefinic bonds.
Whether the oil is of animal or plant origin, it is the fatty acid residues Ri which give the triglycerides their properties.
In one preferred embodiment of the invention, the fatty acid residues Ri in the triglycerides constituting the oil contain more than 60%, and preferably more than 80%, of saturated or monounsaturated chains R1, R2 or R3.
Among the triglycerides containing fatty acid residues of monounsaturated chains Ri, castor oil is preferred.
According to the invention, in the triglycerides of formula (I), the fatty acid residues of chains Ri consist of at least 60% of Ri derived from oleic acid and preferably more than 77% of Ri derived from oleic acid. Oils with a high content of oleic acid, such as, for example, oleic sunflower oil, show excellent resistance to oxidation at high temperatures, which allows the amounts of antioxidants to be reduced, and consequently allows the cost price of the drilling mud to be reduced.
The preferred oleic plant oils are oleic rapeseed oil and oleic sunflower oil.
These plant oils, which are reputed to be relatively unstable for bottom drilling, are sufficiently stable under the conditions in which a surface drilling is carried out, i.e. about 100xc2x0 C. for a sufficiently short period of time, and without an alkaline reserve promoting the hydrolysis of the triglycerides of which they are composed.
In the oily phase according to the invention, the diluent acts not only to lower the viscosity of the animal or plant oil, but also to improve the lubricant properties and the stability of the resulting emulsion of which the surface sludge is composed.
Since there is little or no possibility of hydrolysis of the ester function under the conditions in which the surface sludge is used, these diluents can be chosen from synthetic esters of linear or branched, saturated or unsaturated aliphatic carboxylic acids comprising from 6 to 24 carbon atoms per chain and of alkanols comprising from 1 to 4 carbon atoms per chain.
These esters are synthesized by esterification, using an alkanol comprising from 1 to 4 carbon atoms per chain, of carboxylic acids obtained by hydrolysis of triglycerides from natural animal or plant oils. It is also possible to synthesize these esters by direct transesterification of the natural triglycerides using an alkanol.
An animal or plant oil or alternatively hydrocarbon-based fractions rich in triglycerides of formula (I), containing more than 60% and preferably more than 80% of saturated or monounsaturated chains R1, R2 or R3, can be chosen as starting material in order to prepare the esters constituting the diluent according to the invention.
In one preferred embodiment of the invention, the synthetic esters constituting the diluent of the invention are formed from plant or animal oils from the group consisting of sunflower oil, copra oil, soybean oil, rapeseed oil, castor oil, palm oil, palm kernel oil, groundnut oil, linseed oil, olive oil, corn oil, babassu oil, coconut oil and tallow oil.
An alkanol chosen from the group consisting of isopropanol, ethanol, propanol, butanol and methanol is preferably used for the esterification.
In one preferred embodiment of the invention, the diluent is synthesized from oils chosen from rapeseed oil, copra oil, sunflower oil and castor oil and from an alcohol, preferably isopropanol.
Bottom sludges chosen from the reverse emulsions obtained with mineral or organic oils which are stable up to 250xc2x0 C. and above, and non-toxic muds whose cuttings can be returned and stored onshore on account of their low volume, are used in the second part of the drilling. The term xe2x80x9cmineral and organic oilsxe2x80x9d refers to alkylated linear olefins, mono-xcex1-olefins (MAO), poly-xcex1-olefins (PAO), paraffins, esters, acetals and linear alkylbenzenes (LAB).
Another subject of the invention is a surface sludge which is stable up to about 100xc2x0 C., consisting of from 5 to 30% by volume of water and from 95 to 70% by volume of an oil phase containing a majority of unrefined plant or animal natural oils and a minority of a diluent consisting of esters of these natural oils.